Discovery of bacteria in humans: Sept. 17, 1683

Antonie van Leeuwenhoek, a Dutch merchant who has taught himself how to build a microscope, sends a letter to the Royal Society of London detailing the strange things he has found living inside his mouth. Van Leeuwenhoek explains how, after scraping plaque from his teeth and putting it under his microscope, he was able to see, “with great wonder,” many microscopic organisms that he called “animalcules.”

He writes that they were “very prettily a-moving” and that “the biggest sort had a very strong and swift motion, and shot through the water (spittle) like a pike does through water.” The second sort, he says, “oft-times spun round like a top. . . and these were far more in number."

Leeuwenhoek also describes what he had discovered in the plaque of two old men who had never brushed their teeth in their lives. He observed "an unbelievably great company of living animalcules, a-swimming more nimbly than any I had ever seen up to this time. The biggest sort. . . bent their body into curves in going forwards. . . Moreover, the other animalcules were in such enormous numbers, that all the water. . . seemed to be alive."

Leeuwenhoek’s letter is the first record of bacteria seen living inside the human body.

It’s not the first time that Leeuwenhoek has shared his discoveries with the Royal Society. Ten years earlier, he had first sent the organization a letter detailing his observations of what mold, lice and even bee stings looked like under a microscope. But in 1676, Leeuwenhoek had described something that the members of the Royal Society didn’t believe was possible—he insisted that he had discovered microscopic living things inside pond scum scraped from a lake near his home. They remained dubious until Leeuwenhoek convinced them to see for themselves. And they found that he had been right—there were countless one-cell organisms moving around in the water.

The scientists no longer doubted Leeuwenhoek’s findings--not in 1677 when he became the first person to observe spermatozoa swimming in human sperm, nor in 1682, when he described the nucleus inside the red blood cells of fish, and certainly not the following year when he shared his story about the strange tiny creatures he found inside his mouth.

Roughly 100 years later, in 1877, the Royal Society established the Leeuwenhoek Medal, awarded each decade to the person judged to have made the most significant contributions to the field of microbiology. One of the first winners was Louis Pasteur, who not only demonstrated that fermentation was caused by the growth of bacteria, but also discovered that a process that became known as “pasteurization” could kill bacteria.

It wasn’t until 1876—200 years after Leeuwenhoek first saw bacteria “very prettily a-moving” under his microscope—that a German doctor named Robert Koch proved that those little organisms can cause disease.